In complex optical systems, laser beams can have different input parameters that are to be adapted to narrow output specifications. Specific input parameters of a laser beam, which is to be coupled into an optical system, result from the characteristics of the individual beam sources or of the beam sources in combination with beam guides or further optics placed in the beam, for example. These characteristics differ, for example, from laser type to laser type, from product series to product series, from system (installation) to system. However, as complex optical systems (such as beam sources or beam guides) can rarely be built or operated identically without tolerance, differing characteristics can also arise between individual units of a product series planned as identical units, e.g., from not completely identical or imperfect individuals of beam sources or beam guides. Specific output parameters may arise from special optical requirements and an optical process to be performed by the optical system that receives the laser beam. Analogous to the input parameters, the required output parameters can be different and vary individually. However, output parameters often lie within very narrow tolerances due to the application or the subsequent optics to enable precise laser applications or processes repeatable in an exact manner.
Herein, it is referred to a lateral beam dimension and a divergence of a beam. Essentially, this refers to the second moments of the field or intensity distributions as defined in the standards ISO 13694, ISO 11145 and ISO 11146. However, other appropriate definitions may also be used.
Often one needs to adjust the lateral extent of a laser beam and its divergence in several axes in different ways between beam source, beam guide, and application system. Many systems receiving such a beam require a particularly rotationally symmetrical beam, which the beam sources or the optical systems preceding the receiving system cannot provide with the required accuracy. In some receiving systems, however, a precisely defined non-rotational symmetric form, e.g., elliptical form, astigmatic form, or both of the laser beam is also a target of the adaptation. In general, US 2009/0257118 A1 shows a telescope arrangement for beam shaping which has no variability with regard to beam adaptation due to the pre-selected mirrors, for example.
When, e.g., ultraviolet (UV) laser radiation is generated by non-linear frequency conversion, undesired aberrations in the amplitude and phase of the UV laser radiation can occur within a frequency conversion unit. Such aberrations are compensated, e.g., by optical systems being positioned downstream and designed for correction, see, e.g., US 2012/0120481 A1 and US 2004/0228372 A1. However, optical components of such optical correction systems can experience a gradual influence of the surface, in particular of a surface coating, by the UV laser radiation, which can be detrimental to the function of the overall system.
Furthermore, DE 10 2010 003591 A1 discloses a frequency conversion set-up to compensate for a difference in the propagation directions in a non-linear crystal. Therein, a beam offset of two input beams is effected by passing a lens with a relatively small offset relative to the beam axis, with a tilt, or both. As only a minor beam offset is required, an excessively strong ellipticity of the incident laser beams is avoided. Furthermore, U.S. Pat. No. 8,422,119 B1 discloses a combination of a non-linear crystal for frequency conversion and a cylindrical lens arranged in front of the non-linear crystal to produce a specific beam shape in the non-linear crystal.
In addition, EP 1 237 034 B1 discloses an optical system with electronic spot size control and focus control, US 2012/0032065 A1 discloses a dynamic wavefront control of a frequency-converted laser system, and U.S. Pat. No. 5,745,296 discloses a multi-beam writing device.